The present invention relates to an improved method of forming a glass preform from which optical fibers can be made. More particularly, this invention relates to the processing of optical waveguide core preforms to form optical waveguide canes or rods from which overclad preforms can be produced for drawing into optical waveguide fibers. This invention also relates to the manufacture of optical waveguide components, such as couplers and amplifiers, which require high quality glass.
One known outside vapor deposition (OVD) process for producing waveguide preforms involves forming a core soot preform by applying a first soot coating of uniform or radially varying composition to the surface of a rotating cylindrical mandrel or starting member. After formation of a core soot preform, the mandrel is typically removed, and the soot preform is consolidated and sintered to provide a consolidated core glass blank having an aperture therethrough. The core glass blank can then be drawn into an optical waveguide core rod, which is also referred to as a cane. The optical waveguide core rod can be overclad to form an overclad preform which is then drawn into optical waveguide fiber. This process, which involves separate manufacturing steps for the core preform and the overclad preform, enables better control of the refractive index profile of the resulting fiber and more efficient and lower cost manufacturing of optical fiber. For the manufacture of high quality preforms, particularly core preforms, the reduction and preferably the elimination of inclusions is of critical importance. Inclusions in the preform may be in the form of crystallites or gas bubbles, which are also referred to as seeds. Inclusions in the preform, particularly the core preform, adversely affect the quality of the fiber drawn therefrom. For example, inclusions can reduce fiber strength and increase attenuation in the waveguide fiber. An unacceptably high level of inclusions in a preform may cause the preform to be rejected before fiber is drawn therefrom, increasing manufacturing cost.
During the manufacture of the core glass blank, inclusion formation may occur during the step in which cane is drawn from a core preform. The problem of inclusion formation is particularly problematic with glass compositions containing an oxide such as aluminum oxide. U.S. Pat. Nos. 5,693,115 and 5,330,548 have suggested eliminating gas bubble inclusions in waveguide preforms by heating the preform in multiple stages or by heating the preform in a vacuum atmosphere or reduced atmosphere. Elimination of inclusions formed in a cane during drawing from the core preform, however, would require an additional processing step performed after the cane is drawn from core preform.
It would be advantageous to provide a process that eliminates or at least substantially reduces inclusions formed during the cane drawing process. Such a process would eliminate the need to perform additional processing steps to eliminate inclusions formed during the step of drawing the cane.
Accordingly, the present invention generally provides a method of forming a glass article such as a waveguide preform that is substantially free of inclusions such as gas bubbles and crystallites. The method comprises the steps of providing an elongated, consolidated glass preform having a longitudinal aperture therethrough and drawing the elongated, consolidated preform in at least two steps to provide a rod. The two-step draw process comprises a first step of heating at least one end of the elongated, consolidated glass preform at a first temperature and drawing the preform to provide a reduced diameter preform. The second step of the draw process involves a second step of heating at least one end of the reduced diameter preform at a second temperature and drawing the reduced diameter preform to provide a rod having a diameter less than the reduced preform. The method of the present invention also includes closing one end of the aperture and evacuating the aperture.
Preferably, the step of evacuating the aperture is performed during the second step of the two-step draw process. According to one embodiment of the present invention, during the first step air bubble inclusions coalesce to form continuous elongated air bubble inclusions. The method of the present invention is particularly useful for making optical waveguide fibers made from a glass comprising Al2O3, GeO2 and SiO2, which can be used as a host glass containing dopants such as erbium for making optical amplifiers.
Several important advantages will be appreciated from the foregoing summary. The principal advantage of the present invention is providing a waveguide core preform rod that is substantially free of inclusions, such as air bubbles and crystallites. A waveguide core preform free of such inclusions will require fewer processing steps to produce waveguide fiber, and waveguide fiber drawn therefrom will contain less scattering sites and thus have a lower attenuation. Additional features and advantages of the invention will be set forth in the description which follows, and may be learned by practice of the invention.